This invention relates to a digital automatic gain control apparatus for controlling the signal so that the signal is at a fixed level in processing the digital signal derived from A-D conversion of video signal in a television set, a video cassette recorder (hereinafter referred to as a VCR), etc.
There has been a growing requirement for digital processing of the video signal. The automatic gain controller (hereinafter referred to as the AGC circuit) has conventionally been a type in which analog data processing is performed. However, the needs has been mounting for AGC circuits in which digital data processing is performed. In digital data processing for gain control, as in analog data processing for gain control, an input signal (video signal) is passed through a variable gain amplifier, a difference between the output and a predetermined reference level is calculated, the difference is integrated, and the result is fed back to the variable gain amplifier, in which the gain is thereby controlled to maintain the output signal at a fixed level.
When the input signal is a signal such as a color signal having added thereto a burst signal which represents the reference level, the burst signal for a fixed period of time is extracted, the level of the extracted burst signal is averaged, the average level obtained is fed back to the variable gain amplifier in which the gain is thereby controlled to maintain the burst signal of the output signal (color signal) at a fixed level.
In such an arrangement, when the level of the input signal is low in relation to the dynamic range of the input to the A-D converter which converts the analog input signal into a digital signal, the output signal of the variable gain amplifier is kept at a fixed level as described above; however, a number of bits of quantization to be allotted to the input signal at the A-D converter is reduced, thus increasing the quantization noise and deterioration the signal-to-noise ratio. Therefore, it is conventionally necessary to increase the number of bits of quantization in the A-D converter or amplify the input signal by providing an analog AGC circuit at the stage previous to the A-D converter in order to prevent the signal-to-noise ratio from deteriorating even if the level of the input lowers in the A-D converter.
When the input signal is a color signal and the burst signal included therein is kept at a fixed level, if the input signal has a narrow band as the reproduced signal of the VCR (for example, when NTSC video signals are all subjected to FM modulation, the band width expands as wide as 15 MHz, but the VCR is not arranged to be capable of magnetic recording with frequencies higher than 8 MHz), the envelope of the burst signal is not in a rectangular waveform and distorted. Therefore, if the position at which the burst signal is to be extracted to detect the burst signal level deviates from the burst signal, there arises an error in the amplitude value (level) detected, thereby varying the level of the output signal of the variable gain amplifier, which has been a disadvantage of the conventional arrangement.